The proposed research will seek to determine what restraints are imposed on embryonic myocardial cells by the presence of the neuronal elements which innervate them. Embryonic myocardial cells have the potential to become autorhythmic and to function as pacemaker cells in the heart. However, this potential is rarely manifested by mature myocardial cells for they either completely loose their autorhythmic capability or else it is suppressed in some way during development. We plan to investigate the possibility that the autorhythmic behavior of embryonic myocardial cells is suppressed by some influence placed on them once they are innervated by cardiac neurons. The system to be studied is that of the horseshoe crab Limulus polyphemus and the American lobster. The horseshoe crab heart is myogenic when it begins to beat, but upon subsequent innervation by the cardiac ganglion this myogenicity disappears and the myocardial cells are driven by autorhythmic neurons in the ganglion. We shall employ electrophysiological recording techniques and electron microscopical procedures to follow the changes that occur in myocardial cell properties during the transformation from the myogenic to the neurogenic system. In this way we hope to better understand what are the constraints placed on myocardial cells during development and on which properties of the cells are the constraints imposed. In addition, the results may provide new insights into the underlying causes of ectopic foci.